The invention relates generally to the manufacture of prepregs, that is, partially cured sheets of fabric-reinforced thermoset resins. Most commonly, these are woven fabrics of glass fiber which are coated with epoxy resins and then partially cured. Such prepregs are used to fabricate laminates for the production of the printed circuit boards found in countless consumer products.
The quality of the prepregs is determined among other things by the uniformity of the product. One important measurement is the degree to which the resins have been cured, since they are intentionally only partly cured. The prepregs are usually produced on large pieces of equipment ("treaters") which continually apply resins to fiberglass fabric and then partially cure it by heating. The rolls of fabric may be about 3.2 to 4.2 feet (1-1.3 m) wide and up to about 3000 to 7800 feet (914-2377 m) long. It will be obvious that significant variations in the amount of resin applied to the fabric could occur over the time that such large rolls of fabric are handled. If the amount of the resin varies, then variations in the degree of cure may result even if otherwise the conditions are uniform. In addition, the degree to which the resins are cured may vary depending on the amount of heat received by the wide "web" of fiberglass as it passes through the treater.
Another source of variation in the quality of the prepregs is the composition of the resins themselves for they typically comprise mixtures of epoxy compounds along with curing agents. If the quality of the raw materials varies, then the degree of cure achieved may vary as well. Those familiar with the art will understand that such variations in quality do occur, making uniformity difficult to obtain.
Still another source of non-uniformity of prepregs is the variations in the fabric. If the fabrics are not uniform the amount of resin applied will be affected and the degree of cure of the resin in the finished prepreg as well.
While infrared spectroscopy has been applied to measuring the degree of cure of polymers such as epoxy resins, continuous on-line application to large size commercial prepregging equipment is a difficult task and not heretofore practiced successfully so far as is known. The reasons for this are many, including all the inherent variations in the process described above and the difficulties associated with on-line application of an analytical instrument.
The degree to which prepregs are cured is the subject of U.S. Pat. No. 5,142,151, which discusses a method of determining the degree of resin cure and its application to commercial equipment for coating fabrics with thermoset resins and then partially curing them. The method used involves the measurement of the degree of resin cure by infrared spectroscopy, particularly using the type of equipment called FTIR (Fourier Transform InfraRed).
The process generally described in U.S. Pat. No. 5,142,151 involves passing a beam of infrared light through or against a moving glass fiber web which is coated with a partially cured resin and then determining the degree of cure by the absorption centered at certain wavelengths of infrared light. Wavelengths were identified which were associated with the reactive epoxy groups and with methyl groups (which do not react) and could be used to determine the amount of resin present and the degree of epoxy cure. The epoxy and methyl groups were shown to absorb in the regions centered at about 4529 and 4060 wavenumbers (cm.sup.-1). The FTIR instrument measures the amount of light absorbed in the regions centered about the selected wavelengths and then mathematically manipulates the data to calculate the degree to which the resins have been cured and the amount of resin present. It is shown in the '151 patent that the absorbance at the chosen frequency can be related to the gel time (a standard test for the degree of cure of epoxy resins, IPC Test Method 2.3.18) or the flow testing method (another standard test, IPC Test Method 2.3.17). Such traditional methods available in the art for measuring the curing characteristics of resin formulations are imprecise and require a consistency of technique which is difficult to achieve.
The disadvantage of using FTIR is that it is relatively slow so that continuous traverse of a moving prepreg sheet is impractical. As shown in U.S. Pat. No. 5,142,151 with multiple light sources and receivers it could be implemented, although costly. However, only specific locations across the web could be sampled.
Once the degree of cure has been determined it is then feasible to make adjustments in the prepreg equipment to advance or retard the cure and to correct for variations in the equipment performance to the extent that the equipment permits. A number of such adjustments are illustrated in the '151 patent, including changing the temperature to which the coated glass fiber web is exposed, adjusting the speed of the web, correcting the amount and uniformity of the resin on the fabric, and changing heat input from the local heating elements.
The present invention represents improvements to the general process disclosed in the '151 patent which make possible production of prepregs of a uniformity not previously available. In particular, the invention relates to the use of continuous on-line monitoring of a moving web of prepreg using infrared spectroscopy.